When a gas stream is to be uniformly distributed in a passage which increases in cross-sectional area and in which the diffuser angle exceeds a certain lower limit, it is necessary to install distributing devices. This is particularly necessary in the gas ducts leading to electrostatic precipitators. In such cases, the cross-sectional area may have to be increased in a ratio of up to 1:15. Such gas-distributing devices usually consist of perforated sheet metal panels forming sheet metal walls, in which a large number of surface elements have been punched out and/or lanced to provide a certain ratio of gas flow area to retaining area. The perforated sheet metal panels are arranged to extend substantially at right angles to the gas flow and in passages having a relatively large size are composite. In that case, the individual sheet metal panels are secured by connecting elements to a separate carrying structure.
In a known gas-distributing device, the perforated sheet metal panels are flanged in L-shape on two mutually opposite sides and are flanged in channel-shape on the other two sides, the supporting elements consisting of an I-shaped and channel-shaped rolled section, and only pluggable elements are provided to connect the perforated sheet metal panels and the rolled sections (U.S. Pat. No. 4,207,083).
It has been found that in this known gas-distributing device the velocity of distribution of the cross-section is relatively strongly disturbed by the carrying elements and that this adversely affects the separation performance of the electrostatic precipitator. Particularly when it is necessary to provide a plurality of such gas-distributing devices with their carrying elements in series in the direction of flow, high values of the relative standard deviation S will be obtained. That relative standard deviation S is defined as follows: ##EQU1## wherein vm=mean gas velocity in the cross-section in which the measurement is performed.
vi=local gas velocity in equal-are elements of the cross-section in which the measurement is performed PA1 n=number of elemental areas in which a measurement is performed.